1) Field of the Invention
The present invention relates to an apparatus for and a method of embedding and detecting digital watermark information in an image.
2) Description of the Related Art
Digital watermarking for embedding another kind of information by adding to digital contents a small modification such that deterioration is indistinctive is heretofore known. As an application example of the digital watermarking, examples of embedding information for identifying a writer and information for certifying the ownership are known. Other than this, application techniques of digital watermark are proposed for copy control, finger printing, authentication of contents, and broadcast monitoring.
A digital watermark embedding method is also proposed. For example, a method of embedding watermark information both in the space direction and the time direction is known. According to this method, even when a part of the image is cut out, watermark information embedded in other parts can be extracted (see for example, Japanese Patent Application Laid-Open No. 2003-143559).
As another example, when embedding repetitive watermarks in which a unit watermark is repeated vertically and horizontally, a method is known in which the unit watermark is shifted by a certain amount in the horizontal direction for each line. According to this method, since repetition of the unit watermark is not constant, there is an effect that an attack against the watermark becomes difficult (see for example, Japanese Patent Application Laid-Open No. 2000-151984).
In the above digital watermark embedding method, however, when one of the watermark information is specified, there is a possibility of attack against the watermark information. Further, when a part of the image is cut out, there is a problem in that the watermark information cannot be detected, unless at least one of various kinds of information embedded in the image is completely left.